Servocontrolled transmissions, which are structurally similar to a manual servocontrolled transmission of the traditional type except for the fact that the pedal of the clutch and the gear selection lever operated by the driver are replaced by corresponding electrical or hydraulic servo controls, are increasingly widespread. By using a servocontrolled transmission the driver must only send the control unit of the transmission the command to up-shift or down-shift and the control unit of the transmission autonomously carries out the change of gear by acting both on the engine and on the servo controls associated to the clutch and gearbox.
Generally, the servo control of the gearbox and the servo control of the clutch are of the hydraulic type and comprise corresponding hydraulic actuators connected to a common hydraulic circuit which comprises a storage tank containing the control fluid used by the hydraulic actuators (typically oil) at an ambient pressure, a hydraulic accumulator containing pressurised control fluid, an electrical pump which picks up the control fluid from the storage tank and supplies the pressurised control fluid to the hydraulic accumulator, and a number of electrovalves, which are adapted to selectively connect chambers of the hydraulic actuators to the storage tank and to the hydraulic accumulator. In other terms, in the known applications which are commercialised at present, the pump of the hydraulic circuit is actuated by an electrical motor (electrical pump assembly), which is fed by the motor vehicle battery and may therefore work independently of the state of motion of the thermal engine.
The use of an electrical pump assembly for the hydraulic circuit has several drawbacks: the electrical motor may be liable to ruptures (reduced reliability in time) especially in heavy use conditions, the electrical pump assembly is noisy, and finally the electrical pump assembly has installation criticalities as it requires an appropriate cooling of the electrical motor.
To solve the above described drawbacks, it has been suggested to eliminate the electrical motor and operate the hydraulic circuit pump by obtaining the motion directly from the thermal motor.
For instance, patent application GB2339606A describes the possibility of operating a pump of a hydraulic circuit of a transmission system obtaining the motion directly from the thermal engine; specifically GB2339606 provides the use of an auxiliary electrical motor when the motion obtained from the thermal engine is insufficient.
Also U.S. Pat. No. 5,474,428A1 describes the possibility of operating a pump of a hydraulic circuit of a transmission system obtaining the motion directly from the thermal engine or, as an alternative, by using an electrical motor; specifically, the oil supplied by the pump may be used both for lubricating the transmission and for operating the transmission during a change of gear.
However, the above described solutions which are known for eliminating the electrical motor and operating the pump of the hydraulic circuit thus obtaining the motion directly from the thermal engine are not completely satisfactory, because they provide in any case the use of a small auxiliary electrical motor which is operated when the motion obtained from the thermal engine is insufficient; accordingly, such known solutions are complicated, expensive and voluminous.